On 11/28/18 8:27 PM, Ming Lei wrote:

On Wed, Nov 28, 2018 at 08:13:43PM -0700, Jens Axboe wrote:

quoted

On 11/28/18 7:51 PM, Ming Lei wrote:

quoted

On Wed, Nov 28, 2018 at 07:19:09PM -0700, Jens Axboe wrote:

quoted

On 11/28/18 6:23 PM, Ming Lei wrote:

quoted

On Tue, Nov 27, 2018 at 07:34:51PM -0700, Jens Axboe wrote:

quoted

On 11/27/18 7:10 PM, Ming Lei wrote:

quoted

On Mon, Nov 26, 2018 at 09:35:53AM -0700, Jens Axboe wrote:

quoted

We need this for blk-mq to kick things into gear, if we told it that
we had more IO coming, but then failed to deliver on that promise.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 drivers/block/virtio_blk.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 6e869d05f91e..b49c57e77780 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c

@@ -214,6 +214,20 @@ static void virtblk_done(struct virtqueue *vq)
 	spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
 }
 
+static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
+{
+	struct virtio_blk *vblk = hctx->queue->queuedata;
+	int qid = hctx->queue_num;
+	bool kick;
+
+	spin_lock_irq(&vblk->vqs[qid].lock);
+	kick = virtqueue_kick_prepare(vblk->vqs[qid].vq);
+	spin_unlock_irq(&vblk->vqs[qid].lock);
+
+	if (kick)
+		virtqueue_notify(vblk->vqs[qid].vq);
+}
+
 static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 			   const struct blk_mq_queue_data *bd)
 {

@@ -638,6 +652,7 @@ static void virtblk_initialize_rq(struct request *req)
 
 static const struct blk_mq_ops virtio_mq_ops = {
 	.queue_rq	= virtio_queue_rq,
+	.commit_rqs	= virtio_commit_rqs,
 	.complete	= virtblk_request_done,
 	.init_request	= virtblk_init_request,
 #ifdef CONFIG_VIRTIO_BLK_SCSI

-- 
2.17.1

If .commit_rqs() is implemented, virtqueue_notify() in virtio_queue_rq()
should have been removed for saving the world switch per .queue_rq()

->commits_rqs() is only for the case where bd->last is set to false,
and we never make it to the end and flag bd->last == true. If bd->last
is true, the driver should kick things into gear.

OK, looks I misunderstood it. However, virtio-blk doesn't need this
change since virtio_queue_rq() can handle it well. This patch may introduce
one unnecessary VM world switch in case of queue busy.

Not it won't, it may in the case of some failure outside of the driver.

If the failure is because of out of tag, blk_mq_dispatch_wake() will
rerun the queue, and the bd->last will be set finally. Or is there
other failure(outside of driver) not covered?

The point is to make this happen when we commit the IOs, not needing to
do a restart (or relying on IO being in-flight). If we're submitting a
string of requests, we should not rely on failures happening only due to
IO being going and thus restarting us. It defeats the purpose of even
having ->last in the first place.

OK, it makes sense.

quoted

quoted

quoted

The only reason that virtio-blk doesn't currently hang is because it
has restart logic, and the failure case only happens in the if we
already have IO in-flight.

Yeah, virtqueue_kick() is called in case of any error in
virtio_queue_rq(), so I am still wondering why we have to implement
.commit_rqs() for virtio-blk.

It's not strictly needed for virtio-blk with the restart logic that it
has, but I think it'd be nicer to kill that since we have other real use
cases of bd->last at this point.

quoted

quoted

quoted

IMO bd->last won't work well in case of io scheduler given the rq_list
only includes one single request.

But that's a fake limitation that definitely should just be lifted,
the fact that blk-mq-sched is _currently_ just doing single requests
is woefully inefficient.

I agree, but seems a bit hard given we have to consider request
merge.

We don't have to drain everything, it should still be feasible to submit
at least a batch of requests. For basic sequential IO, you want to leave
the last one in the queue, if you have IOs going, for instance. But
doing each and every request individually is a huge extra task. Doing
IOPS comparisons of kyber and no scheduler reveals that to be very true.

quoted

quoted

quoted

I wrote this kind of patch(never posted) before to use sort of
->commits_rqs() to replace the current bd->last mechanism which need
one extra driver tag, which may improve the above case, also code gets
cleaned up.

It doesn't need one extra driver tag, we currently get an extra one just
to flag ->last correctly. That's not a requirement, that's a limitation
of the current implementation. We could get rid of that, and it it
proves to be an issue, that's not hard to do.

What do you think about using .commit_rqs() to replace ->last? For
example, just call .commit_rqs() after the last request is queued to
driver successfully. Then we can remove bd->last and avoid to get the
extra tag for figuring out bd->last.

I don't want to make ->commit_rqs() part of the regular execution, it
should be relegated to the "failure" case of not being able to fulfil
our promise of sending a request with bd->last == true. Reasons
mentioned earlier, but basically it's more efficient to commit from
inside ->queue_rq() if we can, so we don't have to re-grab the
submission lock needlessly.

I like the idea of separate ->queue and ->commit, but in practice I
don't see it working out without a performance penalty.

Thanks for your detailed explanation, this patch looks fine:

Reviewed-by: Ming Lei <redacted>

Thanks Ming.

-- 
Jens Axboe